Redwoods Record Ancient Ocean Climate

The tallest trees on the planet, the coastal redwood, trapped a record of the Pacific Ocean within their ancient wood.

Centuries of fog from the ocean left a chemical signature in the rings of the massive redwoods (Sequoia sempervirens) that was distinct from the chemicals left by rainwater. A recent study identified that difference and used it to reconstruct the climate patterns of the past 50 years. The same method could produce a Pacific Ocean weather report stretching back to the Roman empire.

“Redwoods are restricted to a very narrow strip along the coastline,” co-author Jim Johnstone said in a press release. “They’re tied to the coastline, and they’re sensitive to marine conditions, so they actually may tell you more about what’s happening over the ocean than they do about what’s happening over land.”

Coastal redwoods can live for nearly 2,000 years. However, until now, the erratic patterns of the trees’ rings limited their usefulness in reconstructing climate from the size of growth rings, a technique known as dendrochronology. Coastal redwoods may grow incomplete rings that don’t always complete the circle around the trunk. This makes it difficult to count the rings to learn the trees’ age or measure the rings to learn about the weather in each year of the trees’ lives

Instead of counting and measuring rings, a team of scientists, measured the levels of different forms of oxygen trapped in the wood. Oxygen in the air holds more of the lighter-weight form, or isotope, of oxygen, O-16, with smaller amounts of heavier O-18.

When water evaporated off of the ocean, some of the water rains back down onto the ocean, carrying the heavier O-18 molecules with it. The rain that reaches land carries more O-16.

Fog, on the other hand, evaporates from the ocean and blows directly onto the trees where it condenses and drips down to water the trees. Since fog doesn’t condense until it reaches the trees, it carries a higher percentage of O-18.

The redwoods sucked up the water from both rain and fog. The ratio of the different types of oxygen corresponded to the varying amounts of fog and rain over the years. For example, a foggy yet dry summer would result in more O-18. Similarly, variations in carbon isotopes in the redwoods reflected changes in growth related to temperature and total moisture in the environment.

For this study, scientists observed only the past 50 years of climate records stored in the redwoods. The chemical clues in the trees matched weather records for the region. By extending the analysis of redwoods back further, a time line of centuries of conditions in the Pacific may become available. This long-term record may help scientists figure out what changes in the ocean are due to natural variability and which are a result of human activities.

The study was published in the Journal of Geophysical Research: Biogeosciences.